EARTH  SCIENCES  LIBRARY 


Preliminary  Report  of  the  State  Earthquake 
Investigation  Commission 


v 


PRELIMINARY  REPORT  OF  THE  STATE  EARTHQUAKE 
INVESTIGATION   COMMISSION, 


BERKELEY,  May  31,  1906. 

To  His  Excellency  George  C.  Pardee, 

Governor  of  California. 

SIR:— 

In  response  to  many  inquiries  that  have  been  addressed  to 
the  State  Earthquake  Investigation  Commission  for  information 
concerning  the  earthquake  which  affected  a  large  part  of  the 
State  of  California  on  April  18,  of  the  current  year,  your  Com- 
mission begs  to  submit  a  preliminary  report  of  its  work  and  of 
some  of  the  more  salient  facts  that  have  come  to  light  in  the 
course  of  its  labors. 

The  Commission  is  acting  under  the  authority  of  your  letter 
of  April  21,  1906,  which  reads  as  follows : — 

"To  WHOM  IT  MAY  CONCERN: 

"The  bearer,  Professor  A.  C.  Lawson,  of  the  State  Univer- 
sity, together  with  Professor  G.  K.  Gilbert  of  the  United  States 
Geological  Survey,  Professor  Fielding  Reid  of  Johns  Hopkins 
University,  Professor  J.  C.  Branner  of  Stanford  University, 
Professor  A.  O.  Leuschner  of  the  State  University,  Professor 
George  Davidson  of  the  State  University,  Professor  Charles 
Burkhalter  of  the  Chabot  Observatory,  and  Professor  William 
Wallace  Campbell,  Director  of  the  Lick  Observatory,  are  hereby 
constituted  a  Committee  of  Inquiry  into  the  earthquake  phe- 
nomena in  all  parts  of  the  State  of  California,  and  the  citizens 
of  the  State  are  hereby  requested  to  cooperate  with  this  com- 
mittee in  all  possible  ways,  affording  them  all  necessary  infor- 
mation and  access  to  the  results  of  the  earthquake  disturbances. 

"(Signed)  GEO.  C.  PARDEE, 

f  l  Governor  of  California. 

"Dated  Oakland,  Gal.,  April  21,  1906." 


870638 


L.c .  The  ^Commission  met  for  purposes  of  organization  on  April 
$}  at' 12  V ''clock  noon,  in  the  rooms  of  the  Faculty  Club,  Univer- 
sity of  California.  Professor  Andrew  C.  Lawson  was  elected 
Chairman,  and  Professor  A.  0.  Leuschner  Secretary.  For  pur- 
poses of  correspondence  it  was  decided  that  the  Commission 
should  be  known  as  the  "State  Earthquake  Investigation  Com- 
mission." It  was  determined  that  the  scope  of  the  work  of  the 
Commission  in  its  preliminary  stages  should  embrace  the  ques- 
tions as  to  the  origin,  the  position,  and  the  character  of  the  dis- 
turbance in  the  earth's  crust  which  gave  rise  to  the  earthquake, 
these  questions  to  be  investigated  by : — 

1.  The  location  of  fault  scarps  and  other  deformations  of 
the  surface  of  the  ground  or  sea-bottom  in  the  region  affected ; 

2.  The  collection  of  time  records  leading  to  the  construction 
of  coseismal  curves ; 

3.  The  collection  of  intensity  records  and  their  classification 
in  a  graded  scale  leading  to  the  construction  of  isoseismal  curves. 

It  was  agreed  that  the  intensity  should  be  considered  in  its 
relation  to  geological  formations  and  structures,  and  in  its  rela- 
tion to  man ;  that  the  relationship  between  types  of  manifestation 
and  the  characters  of  transmitting  formations  should  be  studied 
with  the  object  of  determining  the  cause  of  relative  immunity  of 
certain  localities  and  of  violent  destruction  in  others;  that  in 
the  interpretation  of  intensity  from  the  destruction  of  buildings, 
etc.,  the  character  of  the  structure  should  be  carefully  considered. 

Having  outlined  the  scope  of  its  work,  the  Commission  ap- 
pointed two  committees,  one  on  isoseismals,  Andrew  C.  Lawson 
Chairman,  and  one  on  coseismals,  A.  0.  Leuschner  Chairman. 
The  chairman  of  each  committee  was  authorized  to  name  the 
members  of  his  committee,  the  membership  of  the  committee  not 
being  confined  to  the  Commission.  The  Commission  by  vote  in- 
vited the  cooperation  of  the  San  Francisco  Branch  of  the  Ameri- 
can Association  of  Civil  Engineers  by  asking  them  to  make  avail- 
able to  the  Commission  any  data  they  might  collect  or  results  or 
conclusions  they  might  reach ;  a  similar  service  to  be  rendered  to 
the  Association  by  the  Commission. 

In  view  of  the  alarming  reports  which  were  in  circulation 
after  the  earthquake,  the  Commission  authorized  the  issuance  by 


the  press  of  a  statement  to  the  effect  that  the  times  of  earth- 
quakes cannot  be  predicted,  and  that  any  predictions  such  as 
had  been  current  were  unwarranted ;  that  severe  earthquakes  are 
generally  followed  by  a  number  of  minor  shocks  extending  over 
several  days,  or  even  weeks;  and  that  the  physiographic  condi- 
tions of  the  bay  region  are  such  as  to  preclude  any  serious  dam- 
age from  sea  waves,  popularly  called  the  "tidal  waves,"  due  to 
earthquake  disturbances. 

The  Commission  then  formulated  a  circular,  and  ordered  its 
issue  to  the  press  of  California,  requesting  the  people  of  the 
State  to  cooperate  with  it  in  the  collection  of  data.  The  circular 
appears  as  Appendix  A  to  this  report. 

Professor  Lawson  named  as  the  Committee  on  Isoseismals: 
G.  K  Gilbert,  J.  C.  Branner,  A.  S.  Eakle,  H.  0.  Wood,  R.  S. 
Holway,  F.  E.  Matthes,  C.  E.  Weaver,  J.  N.  LeConte,  H.  W 
Fairbanks,  G.  D.  Louderback,  J.  A.  Reid,  E.  S.  Larsen,  F.  J. 
Rogers,  and  J.  F.  Newsom. 

Professor  Leuschner  named  as  the  Committee  on  Coseismals : 
H.  F.  Reid,  W.  W.  Campbell,  George  Davidson,  Alexander  Mc- 
Adie,  Charles  Burkhalter,  R.  T.  Crawford,  Burt  L.  Newkirk,  A. 
J.  Champreux,  S.  D.  Townley,  and  J.  N.  LeConte. 

At  a  subsequent  meeting  of  the  Commission  it  was  decided 
to  establish  a  third  committee  on  the  geophysics  of  the  earth- 
quake, and  the  Chairman  was  authorized  to  appoint  such  a  com- 
mittee. He  named  Professor  Harry  Fielding  Reid  (Chairman), 
L.  M.  Hoskins,  and  C.  F.  Marvin  as  the  committee.  Dr.  F.  Omori, 
Professor  of  Seismology  in  the  Imperial  University  of  Tokyo,  and 
Professor  Nakamura,  also  of  Tokyo,  were  elected  associate  mem- 
bers of  the  Commission. 

Immediately  after  the  earthquake  and  before  the  appoint- 
ment of  the  Commission,  the  scientific  men  of  the  State  had  been 
active  in  the  investigation  of  the  effects  of  the  earthquake,  and 
the  appointment  of  the  Commission  and  the  plan  of  work  which 
it  formulated  served  to  organize  their  efforts  and  bring  the  re- 
sults of  their  observations  together.  From  the  purely  scientific 
point  of  view,  the  earthquake  of  the  18th  of  April  is  one  of  the 
most  interesting  on  record,  and  in  some  respects  it  is  unique.  A 
complete  study  of  the  phenomena  will  undoubtedly  contribute 


greatly  not  only  to  our  knowledge  of  the  evolution  of  the  earth 's 
crust,  but  also  to  our  knowledge  of  the  means  of  minimizing 
the  calamities  which  arise  from  the  destruction  of  buildings, 
water  pipes  and  other  structures,  when  such  shocks  occur  in  the 
future.  Certain  of  the  studies  which  have  this  important  object 
in  view  fall  within  the  province  of  the  structural  engineer  and 
the  architect,  and  they  have  already  formulated  many  sugges- 
tions dealing  with  principles  and  materials  of  construction  which 
will  be  of  very  great  value  in  the  future  construction  of  build- 
ings, not  only  in  San  Francisco  but  in  all  parts  of  the  country 
which  are  subject  to  earthquake  shocks.  Other  studies  having 
the  same  beneficent  object  in  view  are  more  properly  germain  to 
the  purposes  of  your  Commission.  These  refer  .chiefly  to  the 
relation  between  destructive  effects  and  the  character  of  the 
ground  and  underlying  rocks  upon  which  structures  stand ;  and 
to  this  question  your  Commission  has  given  especial  attention. 
It  has  also  had  constantly  in  mind  the  desirability  of  discovering 
evidence  upon  which  a  judgment  might  be  based  as  to  the  proba- 
i  bility  of  recurrence  of  the  earthquake  in  the  future. 

Having  thus  far  outlined  the  organization  of  the  Commission, 
its  more  important  official  acts,  and  the  scope  and  plan  of  its 
work,  a  word  may  be  said  regarding  the  methods  employed  before 
going  on  to  a  statement  of  the  general  results  attained,  and  of 
the  problems  which  remain  as  yet  to  be  solved.  The  Committee 
on  Coseismals,  being  concerned  with  the  records  of  the  times  at 
which  the  earthquake  was  felt,  has  thus  far  had  to  depend  largely 
upon  correspondence  for  its  information.  The  records  are  of 
two  kinds :  first  those  which  are  based  upon  personal  observations 
at  the  time  of  the  shock,  and  secondly  those  which  were  auto- 
matically registered  by  delicate  instruments  not  only  in  different 
parts  of  the  State  but  also  throughout  the  civilized  world.  For 
the  latter  the  Committee  is  under  obligations  to  seismologists  in 
many  lands  as  well  as  our  own.  For  the  purpose  of  collecting 
the  time  records  in  possession  of  the  citizens  of  the  State  based 
upon  their  personal  observation,  the  Committee  on  Coseismals 
issued  a  supplementary  circular,  a  copy  of  which  is  hereto  ap- 
pended as  Appendix  B.  In  securing  replies  to  the  questions  set 
forth  in  this  circular  from  people  in  the  territory  affected  who 


had  special  opportunities  for  observing  the  time  of  the  earth- 
quake accurately,  the  Committee  on  Coseismals  was  fortunate 
in  being  able  to  avail  itself  of  the  hearty  cooperation  and  assist- 
ance of  various  branches  of  the  public  service  and  of  the  several 
railroad,  telegraph,  and  telephone  companies  operating  in  the 
State,  through  their  -executive  officers.  Such  aid  has  been  nota- 
bly rendered  by  the  officers  of  the  United  States  Weather  Bu- 
reau, the  United  States  Coast  and  Geodetic  Survey,  the  United 
States  Lighthouse  Service,  and  several  officers  of  the  United 
States  Army,  and  of  the  Navy.  The  agents  and  operators  of  the 
Southern  Pacific  Railway,  the  Atchison,  Topeka  and  Santa  Fe 
Railway,  the  California  Northwestern  Railway,  the  Western 
Union  Telegraph  Company,  the  Postal  Telegraph  Company,  and 
the  Pacific  States  Telephone  and  Telegraph  Company,  and  many 
teachers  in  the  high  schools  of  the  State  have  all  been  enlisted  in 
the  work  of  supplying  the  observed  times  of  the  shock  in  various 
parts  of  the  State. 

The  Committee  on  Isoseismals  has  also  been  greatly  assisted 
by  information  contributed  by  citizens  of  the  State  in  response 
to  inquiries  sent  out  through  the  press.  The  numerous  replies 
that  have  been  received  indicate  a  very  widespread  and  intelli- 
gent interest  in  the  phenomena  in  its  purely  scientific  aspects. 
But  the  more  important  collection  of  data  by  this  Committee  has 
been  and  is  being  made  by  field  observation  by  members  of  the 
Committee  and  their  aides. 

RESULTS. 

One  of  the  remarkable  features  of  the  Coast  Ranges  of  Cali- 
fornia is  a  line  of  peculiar  geomorphic  expression  which  extends 
obliquely  across  the  entire  width  of  the  mountainous  belt  from 
Mendocino  County  to  Riverside  County.  The  peculiarity  of  the 
surface  features  along  this  line  lies  in  the  fact  that  they  are  not 
due,  as  nearly  all  the  other  features  of  the  mountains  are,  to 
atmospheric  and  stream  erosion  of  the  uplifted  mass  which  con- 
stitutes the  mountains,  but  have  been  formed  by  a  dislocation  of 
the  earth's  crust,  or  rather  a  series  of  such  dislocations,  in  time 
past,  with  a  differential  movement  of  the  parts  on  either  side  of 
the  plane  of  rupture.  In  general  this  line  follows  a  system  of 


long  narrow  valleys,  or  where  it  passes  through  wide  valleys  it 
lies  close  to  the  base  of  the  confining  hills,  and  these  have  a  very 
straight  trend ;  in  some  places,  however,  it  passes  over  mountain 
ridges,  usually,  at  the  divide  separating  the  ends  of  two  valleys ; 
it  even  in  some  cases  goes  over  a  spur  or  shoulder  of  a  mountain. 
Along  this  line  are  very  commonly  found  abrupt  changes  in  the 
normal  slope  of  the  valley  sides  giving  rise  to  what  are  techni- 
cally known  as  scarps.  These  scarps  have  the  appearance  of  low 
precipitous  walls  which  have  been  usually  softened  and  rounded 
somewhat  by  the  action  of  the  weather.  Small  basins  or  ponds, 
many  having  no  outlet,  and  some  containing  saline  water,  are  of 
fairly  frequent  occurrence  and  they  usually  lie  at  the  base  of  the 
small  scarps.  Trough-like  depressions  also  occur  bounded  on 
both  sides  by  scarps.  These  troughs  and  basins  can  only  be  ex- 
plained as  due  to  an  actual  subsidence  of  the  ground,  or  to  an 
uplift  of  the  ground  on  one  side  or  the  other,  or  on  both  sides. 
The  scarps  similarly  can  only  be  ascribed  to  a  rupture  of  the 
earth  with  a  relative  vertical  displacement  along  the  rupture 
plane.  Frequently  small  knolls  or  sharp  little  ridges  are  found 
to  characterize  this  line  and  these  are  bounded  on  one  side  by  a 
softened  scarp  and  separated  from  the  normal  slope  of  the  valley 
side  by  a  line  of  depression.  In  many  cases  these  features  have 
been  so  modified  and  toned  down  by  atmospheric  attack  that  only 
the  expert  eye  can  recognize  their  abnormal  character ;  but  where 
their  line  traverses  the  more  desert  parts  of  the  Coast  Range,  as 
for  example  in  the  Carissa  Plains,  they  are  well  known  to  the 
people  of  the  country  and  the  aggregate  of  the  features  is  com- 
monly referred  to  as  the  ' '  earthquake  crack. ' ' 

This  line  begins  on  the  north  at  the  mouth  of  Alder  Creek 
near  Point  Arena  and  extends  southeasterly  nearly  parallel  with 
the  coast  line  to  a  point  about  two  miles  below  Fort  Ross,  a  dis- 
tance of  forty-three  miles.  Here  it  passes  outside  of  the  shore 
line  and  is  again  met  with  at  the  point  where  Bodega  Head  joins 
the  mainland.  Thence  it  appears  to  continue  southward  through 
Tomales  Bay  and  Bolinas  Lagoon.  Beyond  Bolinas  Lagoon  it 
passes  outside  of  the  Golden  Gate  and  enters  the  shore  again  at 
Mussel  Rock,  eight  miles  south  of  the  Cliff  House.  From  this 
point  it  is  traceable  continuously  along  the  valley  line  occupied 


by  San  Andreas  and  Crystal  Springs  Lakes,  past  Woodside  and 
Portol a,  over  a  saddle  back  of  Black  Mountain,  thence  along 
Stevens  Creek  Canon,  passing  to  the  southwest  of  Table  Moun- 
tain and  Congress  Springs  to  the  vicinity  of  Wrights,  on  the 
narrow-gauge  railway  between  San  Jose  and  Santa  Cruz.  From 
Wrights  it  continues  on  in  the  same  course  through  the  Santa 
Cruz  Mountains  to  the  point  where  the  Southern  Pacific  Rail- 
way crosses  the  Pajaro  River  near  Chittenden.  From  the  cross- 
ing of  the  Pajaro  the  line  extends  up  the  valley  of  the  San  Be- 
nito  River,  across  the  eastern  portion  of  Monterey  County,  and 
thence  follows  the  northeastern  side  of  the  valley  of  the  San 
Juan  River  and  the  Carissa  Plains  to  the  vicinity  of  Mount  Pinos, 
in  Ventura  County.  The  line  thus  traced  from  Point  Arena  to 
Mount  Pinos  has  a  length  of  375  miles,  is  remarkably  straight, 
and  cuts  obliquely  across  the  entire  breadth  of  the  Coast  Ranges. 
To  the  south  of  Mount  Pinos  the  line  either  bends  to  the  east- 
ward following  the  general  curvature  of  the  ranges  or  is  paral- 
leled by  a  similar  line  offset  from  it  en  echelon;  for  similar  fea- 
tures are  reported  at  the  Tejon  Pass  and  traceable  thence  though 
less  continuously  across  the  Mojave  Desert  to  Cajon  Pass  and 
beyond  this  to  San  Jacinto  and  the  southeast  border  of  the  Colo- 
rado Desert.  The  probability  is  that  there  are  two  such  lines, 
and  that  the  main  line  traced  from  Point  Arena  to  Mount  Pinos 
is  continued  with  the  same  general  straight  trend  past  San  Fer- 
nando and  along  the  base  of  the  remarkably  even  fault  scarp  at 
the  foot  of  which  lies  Lake  Elsinore.  But,  leaving  the  southern 
extension  of  the  line  out  of  consideration  as  somewhat  debatable, 
we  have  a  very  remarkable  physiographic  line  extending  from 
Point  Arena  to  Mount  Pinos  which  affords  every  evidence  of  hav- 
ing been  in  past  time  a  rift,  or  line  of  dislocation,  of  the  earth's 
crust  and  of  recurrent  differential  movement  along  the  plane  of 
rupture.  The  movements  which  have  taken  place  along  this  line 
extend  far  back  into  the  Quaternary  period,  as  indicated  by  the 
major,  well  degraded  fault  scarps  and  their  associated  valleys; 
but  they  have  also  occurred  in  quite  recent  times,  as  is  indicated 
by  the  minor  and  still  undegraded  scarps.  Probably  every  move- 
ment on  this  line  produced  an  earthquake,  the  severity  of  which 
was  proportionate  to  the  amount  of  movement. 


10 


The  cause  of  these  movements  in  general  terms  is  that  stresses 
are  generated  in  the  earth's  crust  which  accumulate  till  they 
exceed  the  strength  of  the  rocks  composing  the  crust  and  they 
find  a  relief  in  a  sudden  rupture.  This  establishes  the  plane  of 
dislocation  in  the  first  instance,  and  in  future  movements  the 
stresses  have  only  to  accumulate  to  the  point  of  overcoming  the 
friction  on  that  plane  and  any  cementation  that  may  have  been 
effected  in  the  intervals  between  movements. 

The  earthquake  of  the  18th  of  April,  1906,  was  due  to  one  of 
these  movements.  The  extent  of  the  rift  upon  which  the  move- 
ment of  that  date  took  place  is  at  the  time  of  writing  not  fully 
known.  It  is,  however,  known  from  direct  field  observations  that 
it  extends  certainly  from  the  mouth  of  Alder  Creek  near  Point 
Arena  to  the  vicinity  of  San  Juan  in  San  Benito  County,  a  dis- 
tance of  about  185  miles.  The  destruction  at  Petrolia  and  Fern- 
dale  in  Humboldt  County  indicates  that  the  movement  on  the 
rift  extended  at  least  as  far  as  Cape  Mendocino,  though  whether 
the  line  of  rift  lies  inland  or  off  shore  in  that  region  is  still  a 
matter  of  inquiry.  Adding  the  inferred  extension  of  the  move- 
ment to  its  observed  extent  gives  us  a  total  length  of  about  three 
hundred  miles.  The  general  trend  of  this  line  is  about  N.  35° 
W.,  but  in  Sonoma  and  Mendocino  counties  it  appears  to  have  a 
slight  concavity  to  the  northeast,  and  if  this  curvature  be  main- 
tained in  its  path  beneath  the  waters  of  the  Pacific  it  would  pass 
very  close  to  and  possibly  inside  of  Capes  Gordo  and  Mendocino. 
Along  the  185  miles  of  this  rift  where  movement  has  actually 
been  observed  the  displacement  has  been  chiefly  horizontal  on  a 
nearly  vertical  plane,  and  the  country  to  the  southwest  of  the 
rift  has  moved  northwesterly  relatively  to  the  country  on  the 
northeast  of  the  rift.  By  this  it  is  not  intended  to  imply  that 
the  northeast  side  was  passive  and  the  southwest  side  active  in 
the  movement.  Most  probably  the  two  sides  moved  in  opposite 
directions.  The  evidence  of  the  rupture  and  of  the  differential 
movement  along  the  line  of  rift  is  very  clear  and  unequivocal. 
The  surface  soil  presents  a  continuous  furrow  generally  several 
feet  wide  with  transverse  cracks  which  show  very  plainly  the 
effort  of  tortion  within  the  zone  of  the  movement.  All  fences, 
roads,  stream  courses,  pipe  lines,  dams,  conduits,  and  property 


11 


lines  which  cross  the  rift  are  dislocated.  The  amount  of  dislo- 
cation varies.  In  several  instances  observed  it  does  not  exceed 
six  feet.  A  more  common  measurement  is  eight  to  ten  feet.  In 
some  cases  as  much  as  fifteen  or  sixteen  feet  of  horizontal  dis- 
placement has  been  observed,  while  in  one  case  a  roadway  was 
found  to  have  been  differentially  moved  twenty  feet.  Probably 
the  mean  value  for  the  amount  of  horizontal  displacement  along 
the  rift  line  is  about  ten  feet  and  the  variations  from  this  are 
due  to  local  causes  such  as  drag  of  the  mantle  of  soil  upon  the 
rocks,  or  the  excessive  movement  of  soft  incoherent  deposits. 
Besides  this  general  horizontal  displacement  of  about  ten  feet 
there  is  observable  in  Sonoma  and  Mendocino  counties  a  differ- 
ential vertical  movement  not  exceeding  four  feet,  so  far  as  at 
present  known,  whereby  the  southwest  side  of  the  rift  was  raised 
relatively  to  the  northeast  side,  so  as  to  present  a  low  scarp  fac- 
ing the  northeast.  This  vertical  movement  diminishes  to  the 
southeast  along  the  rift  line  and  in  San  Mateo  County  is  scarcely 
if  at  all  observable.  .  Still  farther  south  there  are  suggestions  that 
this  movement  may  have  been  in  the  reverse  direction,  but  this 
need,s  further  field  study. 

As  a  consequence  of  the  movement  it  is  probable  that  the 
latitudes  and  longitudes  of  all  points  in  the  Coast  Ranges  have 
been  permanently  changed  a  few  feet,  and.  that  the  stations  occu- 
pied by  the  Coast  and  Geodetic  Survey  in  their  triangulation 
work  have  been  changed  in  position.  It  is  hoped  that  a  reoccu- 
pation  of  some  of  these  stations  by  the  Coast  and  Geodetic  Sur- 
vey may  contribute  data  to  the  final  estimate  of  the  amount  of 
movement. 

The  great  length  of  the  rift  upon  which  movement  has  oc- 
curred makes  this  earthquake  unique.  Such  length  implies  great 
depth  of  rupture,  and  the  study  of  the  question  of  depth  will,  it 
is  believed,  contribute  much  to  current  geophysical  conceptions. 

The  time  of  the  beginning  of  the  earthquake  as  recorded  in 
the  Observatory  at  Berkeley  was  5h  12m  6s  a.m.,  Pacific  stand- 
ard time.  The  end  of  the  shock  was  5h  13m  11s*  a.m.,  the  dura- 
tion being  lm  5s.  Within  an  hour  of  the  main  shock  twelve  minor 
shocks  were  observed  by  Mr.  S.  Albrecht  of  the  Observatory  and 
their  time  accurately  noted.  Before  6h  52m  p.m.  of  the  same 


12 


day  thirty-one  shocks  were  noted  in  addition  to  the  main  dis- 
turbance. These  minor  shocks  continued  for  many  days  after 
April  18,  and  in  this  respect  the  earthquake  accords  in  behavior 
with  other  notable  earthquakes  in  the  past.  The  minor  shocks 
which  succeed  the  main  one  are  interpreted  generally  as  due  to 
subordinate  adjustments  of  the  earth's  crust  in  the  tendency  to 
reach  equilibrium  after  the  chief  movement. 

The  collection  of  time  records  necessarily  proceeds  slowly. 
The  purpose  of  the  coseismal  curves  based  upon  these  records  is 
in  general  two-fold.  In  ordinary  earthquakes  it  is  one  of  the 
means  of  locating  the  seat  of  the  disturbance  when  there  is  no 
surface  manifestation  of  the  rupture  in  the  earth's  crust.  In 
the  present  instance,  however,  the  rupture  has  declared  itself  in 
an  unmistakable  rift  observable  at  the  surface,  and  coseismals 
are  therefore  unnecessary  for  the  determination  of  this  impor- 
tant factor  in  the  general  problem,  so  far  at  least  as  regards  the 
main  disturbance.  It  is  probable,  however,  that  so  radical  a 
change  in  the  equilibrium  of  the  stresses  of  the  earth's  crust 
would  induce  secondary  ruptures  and  consequently  secondary 
earthquakes  closely  associated  with  the  chief  shock.  The  careful 
plotting  of  the  time  records  may,  therefore,  be  useful  in  reveal- 
ing the  location  of  these  secondary  disturbances,  such  for  ex- 
ample as  the  one  which  affected  Southern  California  on  the  after- 
noon of  the  18th  of  April.  The  second  purpose  of  securing  time 
records  is  the  determination  of  the  velocity  of  propagation  of 
the  earth  wave ;  and  the  data  for  this  which  are  likely  to  be  most 
serviceable  are  the  records  obtained  at  various  quite  distant  seis- 
mographic  stations. 

The  destructive  effects  of  the  earthquake  are  in  the  main  dis- 
tributed with  reference  to  the  line  of  rift.  The  exact  limits  of 
the  area  of  destruction  have  not  yet  been  mapped,  but  it  is  known 
to  extend  out  about  twenty-five  or  possibly  thirty  miles  on  either 
side  of  the  rift.  On  the  southwest  side  the  greater  part  of  this 
area  to  the  north  of  the  Golden  Gate  lies  in  the  Pacific.  This 
area  extends  from  Eureka  in  Humboldt  County  to  the  southern 
extremity  of  Fresno  County,  a  distance  of  about  four  hundred 
miles. 

Beyond  this  area  of  destructive  shock  the  earthquake  was  felt 


13 


in  its  milder  manifestations  over  a  wide  territory.  Our  reports 
to  date  show  that  it  was  felt  in  Oregon  as  far  north  as  Coos  Bay 
and  on  the  south  as  far  as  Los  Angeles.  To  the  east  it  was  felt 
over  the  greater  part  of  middle  California  and  eastern  Nevada, 
particularly  along  the  eastern  flank  of  the  Sierra  Nevada.  It 
was  felt  at  Lovelocks,  and  we  have  unconfirmed  reports  of  its 
having  been  felt  at  Winnemucca.  Far  beyond  the  region  within 
which  it  was  apparent  to  the  senses,  however,  the  earth  wave  was 
propagated  both  through  the  earth  and  around  its  periphery; 
and  some  of  the  most  valuable  and  most  accurate  records  of  the 
disturbance  which  we  have  are  those  which  were  registered  at 
such  distant  seismographic  stations  as  Washington,  D.  C. ;  Sitka, 
Alaska ;  Potsdam,  Germany ;  and  Tokyo,  Japan. 

Within  the  area  of  destructive  effects  approximately  400  by 
50  miles  in  extent  the  intensity  varied  greatly.  There  was  a 
maximum  immediately  on  the  rift  line.  Water  pipes,  conduits, 
and  bridges  crossing  this  line  were  rent  asunder.  Trees  were 
uprooted  and  thrown  to  the  ground  in  large  numbers.  Some 
trees  were  snapped  off,  leaving  their  stumps  standing,  and  others 
were  split  from  the  roots  up.  Buildings  and  other  structures 
were  in  general  violently  thrown  and  otherwise  wrecked,  though 
some  escaped  with  but  slight  damage.  Fissures  opened  in  the 
earth  and  closed  again,  and  in  one  case  reported  a  cow  was  en- 
gulfed. A  second  line  of  maximum  destruction  lies  along  the 
floor  of  the  valley  system  of  which  the  Bay  of  San  Francisco  is 
the  most  notable  feature,  and  particularly  in  the  Santa  Rosa  and 
Santa  Clara  valleys.  Santa  Rosa,  situated  twenty  miles  from  the 
rift,  was  the  most  severely  shaken  town  in  the  State  and  suffered 
the  greatest  disaster  relatively  to  its  population  and  extent. 
Healdsburg  suffered  to  a  nearly  similar  degree.  San  Jose,  situ- 
ated thirteen  miles,  and  Agnews,  about  twelve  miles  from  the  rift, 
are  next  in  the  order  of  severity.  Stanford  University,  seven 
miles  from  the  rift,  is  probably  to  be  placed  in  the  same  category. 
All  of  these  places  are  situated  on  the  valley  floor  and  are  under- 
lain to  a  considerable  depth  by  loose  or  but  slightly  coherent 
geological  formations,  and  their  position  strongly  suggests  that 
the  earth  waves  as  propagated  by  such  formations  are  much  more 
destructive  than  the  waves  which  are  propagated  by  the  firmer 


14 


and  highly  elastic  rocks  of  the  adjoining  hill  lands.  This  sug- 
gestion is  supported  by  a  consideration  of  the  destructive  effects 
exhibited  by  towns  and  single  buildings  along  the  same  valley 
line  which  are  situated  wholly  or  partly  on  rock.  Petaluma  and 
San  Rafael,  though  nearer  the  rift  than  Santa  Rosa,  suffered  * 
notably  less,  and  they  are  for  the  most  part  on,  or  close  to,  the 
rocky  surface.  The  portions  of  Berkeley  and  Oakland  which  are 
situated  on  the  alluvial  slope  suffered  more  than  the  foothills, 
where  the  buildings  are  founded  on  rock.  The  same  suggestion 
is  further  supported  from  a  consideration  of  the  zone  of  maxi- 
mum destructive  effect  on  the  southwest  side  of  the  rift.  This 
zone  lies  in  the  Salinas  Valley.  The  intensity  of  destructive  ac- 
tion at  Salinas  was  about  the  same  as  at  San  Jose,  and  the  town 
is  situated  on  the  flood  plain  deposits  of  the  Salinas  River.  Along 
the  banks  of  the  Salinas  River  and  extending  from  Salinas  to 
the  vicinity  of  Gonzales,  so  far  as  our  reports  at  present  show, 
the  bottom  lands  were  more  severely  ruptured,  fissured.,  and 
otherwise  deformed  than  in  any  other  portion  of  the  State.  The 
Spreckels  Sugar  Mill,  situated  on  the  banks  of  the  river,  suffered 
more  severely  probably  than  any  other  steel  structure  in  the 
State.  Santa  Cruz,  on  the  other  hand,  which  is  on  the  same  side 
of  the  rift,  and  at  the  same  distance  from  it,  but  which  is  built 
on  rock  for  the  most  part,  suffered  much  less  damage.  In  the 
northern  counties  along  the  coast  the  most  severe  effects  were 
felt  at  Ferndale,  on  the  south  margin  of  the  flood  plain  of  the 
Eel  River,  and  at  Petrolia,  on  the  bottom  land  of  the  Mattole. 
Fort  Bragg  was  severely  shaken  with  very  destructive  effects, 
but  our  reports  do  not  yet  indicate  the  character  of  the  ground 
upon  which  it  is  situated. 

In  the  facts  which  have  been  cited  we  seem  to  have  warrant 
for  a  generalization  as  to  the  excessively  destructive  effect  of  the 
earth  wave  as  transmitted  by  the  little  coherent  formations  of  the 
valley  bottoms.  But  it  must  be  borne  in  mind  that  by  far  the 
greater  number  of  structures  subject  to  destructive  shock  are  sit- 
uated in  the  valley  lands  and  that  there  has  not  yet  been  time  for 
a  detailed  comparison  of  the  effects  in  the  valleys  with  those  in 
the  hills,  where  the  buildings  are  founded  on  firm  rock  except  in 
a  few  notable  instances. 


15 


The  most  instructive  of  these  instances  is  the  city  of  San 
Francisco,  and  the  facts  observed  there  are  entirely  in  harmony 
with  the  generalization  above  outlined.  In  the  city  of  San  Fran- 
cisco we  may  recognize  for  preliminary  purposes  four  types  of 
ground:  (1)  The  rocky  hill  slopes;  (2)  the  valleys  between  the 
spurs  of  the  hills  which  have  been  filled  in  slowly  by  natural  pro- 
cesses; (3)  tlie  sand  dunes;  (4)  the  artificially  filled  land  on  the 
fringe  of  the  city.  Throughout  the  city  we  have  a  graded  scale 
of  intensity  of  destructive  effects  which  corresponds  closely  to 
this  classification  of  the  ground.  The  most  violent  destruction  of 
buildings,  as  everybody  knows,  was  on  the  made  ground.  This 
ground  seems  to  have  behaved  during  the  earthquake  very  much 
in  the  same  way  as  jelly  in  a  bowl,  or  as  a  semi-liquid  material 
in  a  tank.  The  earth  waves  which  pass  through  the  highly  elastic 
rocks  swiftly  with  a  small  amplitude  seem  in  this  material  to 
have  been  transformed  into  slow  undulations  of  great  amplitude 
which  were  excessively  destructive.  The  filled  in  material  and 
the  swampy  foundation  upon  which  it  rests  behaved,  in  other 
words,  as  a  mass  superimposed  upon  the  earth's  surface,  rather 
than  as  a  part  of  the  elastic  crust  itself.  In  a  less  degree  the 
same  thing  is  true  of  the  sand  dune  areas,  where  the  ground  was 
frequently  deformed  and  fissured.  In  still  less  degree  the  nat- 
urally filled  valleys  between  the  hill  spurs  were  susceptible  to 
this  kind  of  movement,  and  the  destruction  of  buildings  was  cor- 
respondingly less,  but  still  severe,  depending  very  largely  on  the 
character  of  the  buildings,  the  integrity  of  their  construction,  etc. 
In  portions  of  these  valleys,  however,  the  original  surface  of  the 
ground  has  been  modified  by  grading  and  filling,  and  on  the 
filled  areas  the  destruction  was  more  thorough  than  elsewhere  in 
the  same  valley  tracts.  On  the  rocky  slopes  and  ridge  tops,  where, 
for  the  most  part,  the  vibration  communicated  to  buildings  was 
that  of  the  elastic  underlying  rocks,  the  destruction  was  at  a 
minimum.  On  some  of  the  hills  chimneys  fell  very  generally 
and  walls  were  cracked;  on  others  even  the  chimneys  withstood 
the  shock. 

While  this  correlation  of  intensity  of  destructive  effect  ap- 
pears to  hold  as  a  generalization,  there  are  well  known  exceptions 
which  find  their  explanation  in  the  strength  of  the  structures. 


16 


Modern  class  A  steel  structures  with  deep  foundations  appear  to 
have  been  relatively  passive,  while  the  made  ground  in  their 
immediate  vicinity  was  profoundly  disturbed.  Thoroughly 
bonded  and  well  cemented  brick  structures,  on  similarly  deep 
and  solid  foundations,  seem  to  have  been  equally  competent  to 
withstand  the  shock,  except  for  occasional  pier-like  walls  not  well 
tied  to  the  rest  of  the  building.  The  weak  points  in  wooden 
frame  structures  were  in  general  the  faulty  underpinning  and 
lack  of  bracing,  and  chimneys  entirely  unadapted  to  resist  such 
shocks.  With  these  faults  corrected,  frame  buildings  of  honest 
construction  would  suffer  little  damage  beyond  cracking  of  plas- 
ter in  such  a  shock  as  the  18th  of  April,  save  on  the  made  ground, 
where  deep  foundations  and  large  mass  appear  to  be  essential  for 
the  necessary  degree  of  passivity. 

Pipe  lines  and  bridges  crossing  the  rift  line  present  a  pecu- 
liar, if  not  quite  unique,  engineering  problem  which  will  doubt- 
less be  solved  in  the  near  future.  Pipe  lines  on  low  swampy 
ground  or  in  made  ground  are  in  much  greater  danger  of  de- 
struction from  earthquake  shocks  than  those  on  high  ground 
underlaid  by  rock,  except  in  the  immediate  vicinity  of  the  rift, 
where  nothing  could  be  constructed  which  would  withstand  the 
violence  of  the  earth  movement. 

One  of  the  lessons  of  the  earthquake  which  seems  peculiarly 
impressive  is  the  necessity  for  studying  carefully  the  site  of  pro- 
posed costly  public  buildings  where  large  numbers  of  people  are 
likely  to  be  congregated.  In  so  far  as  possible  such  sites  should 
be  selected  on  slopes  upon  which  sound  rock  foundation  can  be 
reached.  It  is  probably  in  large  measure  due  to  the  fact  of  their 
having  such  a  rock  foundation  that  the  buildings  of  the  State 
University,  at  Berkeley,  escaped  practically  uninjured.  The  con- 
struction of  such  buildings  as  our  public  schools  demands  the  most 
earnest  attention  of  the  people  and  of  the  authorities  charged 
with  their  construction.  A  great  many  of  our  schools  proved  to 
be  of  flimsy  construction  and  ill  adapted  to  meet  the  emergency 
of  an  earthquake  shock  of  even  less  severity  than  that  of  the  18th 
of  April. 

The  Commission  in  presenting  this  brief  report  has  had  in 
mind  the  demand  on  the  part  of  the  people  of  the  State  and  of 


17 


the  world  at  large  for  reliable  information  as  to  the  essential 
facts  of  the  earthquake.  It  has,  therefore,  not  presumed  to  en- 
gage in  any  discussion  of  the  more  abstruse  geological  questions 
which  the  event  naturally  raises.  It  leaves  such  discussion  for  a 
more  exhaustive  report  which  can  only  be  prepared  after  the 
campaign  of  data  collection  is  complete,  and  that  may  be  some 
months  hence. 

Very  respectfully  submitted  in  behalf  of  the  Commission, 

ANDREW  C.  LAWSON, 


Chairman. 


A.  0.  LEUSCHNER, 

Secretary. 


18 


APPENDIX  A. 


STATE  EARTHQUAKE  COMMISSION,  BERKELEY, 
CALIFORNIA,  APRIL  25,  1906. 

It  is  of  importance  that  the  citizens  of  the  State  of  California  cooperate 
with  the  State  Earthquake  Commission  in  its  investigation  of  the  recent 
seismic  disturbance.  For  this  purpose  it  is  essential  that  the  Commission 
receive  as  soon  as  possible  information  on  any  or  all  of  the  topics  outlined 
below.  All  communications  should  be  addressed  to  the  State  Earthquake 
Commission,  University  of  California,  Berkeley,  California. 

Give  information  on  the  following: 

1. — Post-office  address;  town,  county,  and  state. 

2. — Place  and  date  of  observation. 

3. — Name  and  address  of  the  observer,  if  other  than  the  writer. 

4. — Give  estimate  of  the  intensity  of  the  earthquake  on  the  Rossi-Forel 
Scale.    The  Rossi-Forel  Scale  as  amended  by  the  Commsssion  is  as  follows: 
I.  Perceptible,  only  by  delicate  instruments. 
II.    Very  slight,  shocks  noticed  by  few  persons  at  rest. 

III.  Slight  shock,  of  which  duration  and  direction  was  noted  by  a 
number  of  persons. 

IV.  Moderate   shock,  reported   by  persons    in    motion;    shaking  of 
movable  objects;  cracking  of  ceilings. 

V.  Smart  shock,    generally    felt;    furniture    shaken;    some    clocks 

stopped;  some  sleepers  awakened. 
VI.  Severe   shock,    general    awakening     of     sleepers;     stopping    of 

clocks;  some  window  glass  broken. 
VII.    Violent  shock,  overturning  of  loose  objects ;   falling  of  plaster; 

striking  of  church  bells;  some  chimneys  fall. 
VIII.  Fall  of  chimneys;  cracks  in  the  walls  of  buildings. 
IX.  Partial  or  total  destruction  of  some  buildings. 
X.  Great  disasters ;  overturning  of  rocks ;  fissures  in  the  surface  of 
the  earth;  mountain  slides. 

5. — Give  any  facts  that  you  can  as  to  the  directions  the  Earthquake 
Waves  seemed  to  travel.  Describe  the  character  of  the  shock,  whether  a 
temblor  or  an  oscillatory  motion,  etc.,  and  whether  you,  yourself,  or  others 
had  any  clear  impressions  as  to  the  direction  in  which  it  was  moving,  the 
facts  on  which  this  impression  was  based  and  whether  people  agreed  as  to 
the  direction. 

6. — Give  also  any  further  particulars  of  interest,  whether  they  are  from 
observation  or  hearsay.  If  any  changes  occurred  in  the  ground,  such  as  de- 
pressions or  elevations  of  the  surface,  fissures,  emissions  of  sand  or  water, 
describe  them  fully.  Character  of  damage  to  buildings.  General  direction 
in  which  walls,  chimneys  and  columns  in  cemeteries  were  overthrown. 
Springs,  wells,  and  rivers  are  .often  notably  affected,  even  by  slight  shocks, 
and  any  information  in  regard  to  such  changes  will  be  valuable. 


19 


7. — State  as  exactly  as  possible  the  time  of  commencement  and  the  duration 
of  each  shock. 

The  exact  time  of  the  beginning  of  a  shock  (to  the  nearest  second),  one 
of  the  most  important  of  all  observations,  is  difficult  to  get  correctly,  be- 
cause of  the  great  velocity  with  which  the  wave  travels,  and  because  the 
watch  or  clock  must  be  immediately  compared  with  a  clock  known  to  be 
keeping  standard  time.  If  several  hours  have  elapsed  before  the  comparison 
is  made,  another  comparison  should  be  made  an  hour  later,  in  order  to  find 
whether  your  timepiece  is  gaining  or  losing.  The  observation  cannot  be 
regarded  as  a  good  one,  unless  it  is  stated  that  this  has  been  done.  Tele- 
graph operators,  railroad  officials,  watchmakers,  etc.,  have  especially  good 
opportunities  for  answering  this  question  correctly,  and  their  cooperation 
is  most  earnestly  solicited. 

If  a  clock  was  stopped,  give  the  exact  time  it  indicated  (and  anything 
known,  as  how  fast  or  how  slow  it  was),  its  position,  the  direction  in  which 
it  was  facing,  and  the  length  of  the  pendulum. 

8. — If  a  shock  was  not  felt  in  your  neighborhood,  although  noticed  at 
places  not  very  far  distant,  do  not  fail  to  answer  the  first  four  questions, 
as  negative  reports  are  of  great  interest  in  defining  the  limits  of  the 
disturbed  area,  etc.  State  also  the  nearest  point  to  your  station  where  the 
shock  was  felt. 

9. — Name  of  the  writer. 

NOTE. — In  replying  to  these  questions,  they  need  not  be  repeated; 
but  the  answers  should  be  numbered  to  correspond  to  the  questions. 


APPENDIX  B. 


COSEISMALS. 

QUESTIONS   REGAKDING   THE    EARTHQUAKE   SHOCKS   ON    AND 
AFTER  APRIL  18,  1900. 

Date  of  shock. 

1.  Town,  County,   State  

2.  Observer     

3.  Observer's  address  and  occupation   

4.  Situation  and  occupation  of  the  observer  at  the  time  ,.... 


Record  for  each  shock  in  separate  column, 


B 


D       E       F 


f>.  Time,  direction,  etc. — 

A.  Time  of  commencement  

B.  Duration    

('.  Direction  of  shock  

(a)   Vertical    

(6)   Horizontal    

(c)  Direction  in  which  objects 
were  overturned  

(d~)  Apparent  direction  of  pro- 
pagation   

D.  Seconds  of  tremulous  motion — 

(a)   Before     

(6)   After     

E.  Nature  of  shock— 

(a)  Did  intensity  gradually  in- 
crease and  then  die 
away?  

(fr)  Were  there  two  or  more 
maxima  of  intensity?  If 
so,  what  was  the  inter- 
val between  them  and 
the  order  of  their  in- 
tensity ?  

(c}  Was  the  principal  disturb- 
ance strongest  near  the 
beginning,  the  middle,  or 
the  end  of  the  series?  .... 

F.  Stopping  of  clocks — 

(a)   Time    

(I)}  Equivalent  length  of  pen- 
dulum   

(c)  Clock  faced   

G.  Sound  phenomena — 

(a)  Was  any  unusual  rumbling 
sound  heard  at  the  time 
of  the  shock,  and,  if  so, 
what  did  it  resemble?  .... 

(fo)  Did  the  beginning  of  the 
sound  precede,  coincide 
with,  or  follow  the  be- 
ginning of  the  shock, 
and , by  how  many  sec- 
onds?   • 

(<?)  Did  the  end  of  the  sound 
precede,  coincide  with, 
or  follow  the  end  of  the 
shock,  and  by  how  many 
seconds?  

(d)  Did  the  sound  become  grad- 

ually louder  and  then 
die  awray?  

(c)  Did  the  instant  when  the 
sound  was  loudest  pre- 
cede, coincide  with,  or 
follow  the  instant  when 
the  disturbance  was 
strongest,  and  by  how 
many  seconds?  

(/")  Did  the  sound  change  in 
character  at  or  about 
the  time  when  the  dis- 
turbance was  strongest?; 

Recorder   .. 


Ecturn  to  State  Earthquake  Commission,  Berkeley,  Cal. 


14  DAY  USE 

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UN  I  0  2005 


LD  21-40m-5,'65 
(F4308slO)476 


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